Abstract:

A system and drive method for compensating intensity variation due to
variation of operating cycles in a dynamically controlled system
comprising a plurality of light emitting devices are provided in this
invention.

Claims:

1. A display apparatus comprising a plurality of light emitting devices
constructed to illuminate an image display panel, a controller regulating
said plurality of light emitting devices according to an input image
information, further comprising:A counter;A first storage device;A second
storage device;Wherein said counter counts the operation of each of said
light emitting devices and provides the accumulated operation cycles of
said light emitting devices;wherein said first storage device records
said accumulated operation cycles of said selected light emitting
devices;wherein said second storage device stores a plurality of
reference levels, each said reference level corresponding to the light
intensity for a light emitting device at a given drive current and after
a given cycles of operation;Wherein said controller sets an intensity
level for a light emitting device according to the accumulated operation
cycles and reference levels in said first and second storage devices by
steps of:(1) determine the drive condition for a light emitting device at
its zero operation cycle, and(2) increase the drive current to compensate
the intensity decrease indicated by the reference level corresponding to
the accumulated cycles of operation of said light emitting device;wherein
said operation cycles is proportional to the time and current of
operating a light emitting device.

2. The display apparatus according to claim 1 wherein said accumulated
operation cycles corresponds to the accumulated operating time; wherein
said accumulated operating time corresponds to the total of time during
which the light emitting device is driven at a given current.

3. The display according to claim 1 wherein said accumulated operation
cycles correspond to the accumulated weighed operating time, wherein the
weighed operating time corresponds to the time during which the light
emitting device is turned on scaled with the drive current; wherein the
time scaled with the drive current corresponds to the time multiplied by
a factor proportional to the drive current.

4. A display apparatus comprising a plurality of light emitting devices
constructed to illuminate an image display panel, a controller regulating
said plurality of light emitting devices according to an input image
information, further comprising:A counter;A storage device;Wherein said
counter records the accumulate operation duty of a selected group of said
light emitting devices;wherein said storage device records said
accumulated operation duty of said selected light emitting
devices;wherein said storage device further stores a plurality of
reference intensity levels corresponding to the time-dependent intensity
of the light emitting device;wherein said controller sets a given
intensity level for a light emitting device by compensating the decrease
of light output of said light emitting device according to the
accumulated operation duty of said light emitting device and the
corresponding decrease in light output of said light emitting device
according to the time-dependent intensity; wherein said accumulated
operating duty is proportional to the sum of all the operating duties
where each operating duty is the time during which a light emitting
device is turned on by a drive current times the amplitude of the drive
current.

5. The display apparatus according to claim 4 wherein said accumulated
operation duty corresponds to the accumulated charge; wherein said
accumulated charge corresponds to the integration of current with time
during which the light emitting device is driven at the said current.

6. The display apparatus according to claim 4 wherein said accumulated
operation duty corresponds to the total charge, which is the total charge
driven through a light emitting device.

7. The display apparatus according to claim 4 wherein said accumulated
operating duty corresponds to the sum of all weighed operating times,
said each weighed operating times is the time during which a light
emitting device is turned on at a drive current times the amplitude of
the drive current.

8. A display apparatus comprising a plurality of light emitting devices
constructed to illuminate an image display panel, a controller regulating
said plurality of light emitting devices according to an input image
information, further comprising:A counter;A storage device;Wherein said
counter counts the accumulated operation duty of a selected group of said
light emitting devices;wherein said storage device records said
accumulated operation duty of said selected light emitting
devices;wherein said storage device further stores a plurality of
reference intensity levels corresponding to intensity of the light
emitting device after a given accumulated operation duty;Wherein said
controller compensates light output of a given light emitting device by
increase the drive current to said light emitting device so that the
increase in light intensity offset the decrease of light output due to
the decrease in light intensity according to the reference intensity.

9. The display apparatus according to claim 8 wherein said accumulated
operation duty corresponds to the accumulated charge; wherein said
accumulated charge corresponds to the integration of current over time
during which the light emitting device is driven at the said current.

[0003]The present invention relates to the drive method and system of
multiple light emitting devices used in as illuminating source for an
image display panel. The invention relates to a scheme that improves on
the non-uniformity caused by the variation of time decay of light
emitting device.

[0004]2. Description of the Prior Art

[0005]Light output of light emitting device, such as organic and inorganic
light emitting diodes (LED), decreases over its operation life. The
decrease of the light intensity follows a fairly predicable decay curve
that may be characterized by a few parameters. In a system where multiple
LEDs are structured as illuminating light source for a display panel,
each LED decays according to its own operation time. Since the turn-on
time for each LED depends on the image stream of a particular location
and color, the LEDs are experiencing different decay cycle. It is
unavoidable that the LEDs are not balanced between different colors and
between different locations, after a certain period of operation. Such
unbalance causes a color shift and various dim or bright spots on a
display image.

[0006]The conventional method of improving the uniformity is to set a
control window of the LED characteristic to select the LEDs of
substantially similar performance and slow decay. This is a procedure
that requires more LED testing, and limits the amount of usable LED.
Furthermore, the variation in decay is caused by dynamic lighting that
turns on LEDs for different durations. The initial screening can hardly
offset the change caused by different operation time.

[0007]Another conventional method is to randomize the image in order to
distribute the duty cycle (the duration during which a device is turned
on) evenly among different LEDs. This is not effective as the
randomization of images can be performed in between regular image
display, but not on the actual image itself. Furthermore, this method
tends to age the LEDs more than the actual operation time and therefore
reduces the usable life of the LEDs.

[0008]The present invention provides a system and method to eliminate the
need for smoothing the LED by over aging the device via randomization,
and provide an effective method to keep LED output uniform across a large
number of LED under dynamic lighting condition.

SUMMARY OF THE INVENTION

[0009]The present invention uses a tracking method and a memory device to
record the LED run time. The system further comprises a reference
intensity levels for various times in the device operation life. Such
information is also stored in a storage device. Each light emitting
device is then reference to the different point of time decay according
to its specific operating run time recorded via a tracking circuitry. The
drive power is then adjusted to match each and every individual light
emitting elements, and thus offsetting any non-uniformity arising from
the time decay.

[0010]In the subsequent discussion, LED is used to represent various types
of light emitting devices having similar characteristics of intensity
decay over operation life.

[0013]The present invention is herein described in detail with reference
to the drawings.

[0014]FIG. 1 provides a preferred embodiment of the schematics diagram of
the present invention, wherein the LEDs are driven by the LED backlight
driver, and the LED backlight driver receives driving signals from the
LED lighting controller. The signals send to the LED driver comprises
information corresponding to the light intensity of a particular LED to
be driven by the driver.

[0015]Preferred embodiments of the control of LED backlight intensity
include amplitude modulation (AM), pulse width (time duration) modulation
(PWM), and pulse width modulation at variable amplitude. In the case of
amplitude modulation, a current level is set for a particular LED to be
driven to an intensity level corresponding to the set current level. The
current level signal is sent to the driver from the controller and the
driver delivers the corresponding current to the LED. In such case, the
decay of the LED after being turned on at this current level is
proportional to the amplitude of the current. Therefore, the information
of the amplitude of the drive current is provided by the controller to a
counter after first being converted via a D-to-A converter. In one
preferred embodiment, each time a light emitting device is driven by a
current I for time t, the weighed time, t×I, is counted by the
counter for this light emitting device. In this embodiment, the weighed
operating time is the time multiplied by the drive current. The is the
scaled operating time. In another preferred embodiment, the weighed
operating time is defined as the time multiplied by a factor proportional
to the drive current. The counter accumulates the counts of weighed
operating time for each LED and stores the counts in a storage device.
The accumulated operating cycles for a light emitting device is the total
of all such weighed operating times. Note that the weighed operating
cycles is the weighed operating time, in an embodiment where time scale
is used as the reference value. A different unit may be used to represent
the accumulated weighed operating cycles.

[0016]Each time when the controller receives image intensity signal from
the LCD/LED signal processor, the controller retrieves the recorded
accumulated operating cycles of each LED stored in the storage, compare
this value with the decay reference level stored in storage 2 and
determines the reference intensity which represents an intensity level
corresponds such a particular accumulated operating cycles. The drive
current is then determined accordingly based on the reference level and
the brightness level required by the image signal.

[0017]In one preferred embodiment, in determining the actual drive signal
and current for a light emitting device, the controller first determines
the intensity (Lt) of the light emitting device at a given accumulated
operating cycles (t) using the given required intensity (L0) and the
corresponding unadjusted drive current setting (I0). This drive current
provides an actual intensity lower than the required setting because the
light emitting device has aged over the time of operation. The controller
raises the current inversely proportional to such decrease in intensity
and thereby offset the decrease in intensity. One embodiment uses a
scaling formula of It=I0×L0/Lt, where It is the actual drive
current to be set at the time, L0 is the targeted intensity and is the
time zero intensity of the light emitting device driven by the current
I0, Lt is the reference intensity of the light emitting device driven by
I0 at the time after accumulated operating cycles t, according to the
reference information stored in the storage and interpolated if t is
between two points.

[0018]In an embodiment of PWM, the intensity is proportional to the
duration of keeping the LED turned on at a given current level. The LED
backlight driver may receive a pulse width signal, or receive the same
current signal as in the AM case and internally convert the current
signal into pulse width. The current is linearly proportional to the
duration. The aging effect of the LED is therefore proportional to the
time or current level signal sent from the controller to the LED
backlight driver. The circuit action of the counter, the method of
extracting the decay point from storage 1, and referencing the level with
storage 2 are similar to the case of AM. If the PWM scheme keeps the
drive current constant, than the weighing of the cycle time may not be
necessary.

[0019]A preferred embodiment for the controller to set the drive signal is
to scale with the reference level. A preferred embodiment of the scaling
procedure is to set the drive signal inversely proportional to the
reference level corresponding to the operation counts of a LED. For
example, when a image signal requires a particular light emitting device
to deliver an intensity level of 124 out of a full brightness level at
256, and this LED has been operated for a certain time so that the
reference level indicates that the full level output of this LED is 200,
the scaling method sets this LED according to (128/200)×256=164 so
that the when driven at level 164, this LED delivers a intensity of level
128 as required.

[0020]The dependence of light intensity of a light emitting device is
often expressed in reference to the accumulated charge. The accumulated
charge is the integral of the current versus time, which is equivalent to
the total charge flowing through the light emitting device. The intensity
decrease with the accumulated charge. In a preferred embodiment, the
controller times each cycle time by the drive current, and the counter
add the value to the accumulated charge and stored in the storage.

[0021]In Another preferred embodiment, both the drive current and the
pulse width vary to achieve a given intensity. The accumulated operating
duty in this embodiment is expressed in term of the sum of all the
operating duties where each operating duty is the time during which a
light emitting device is turned on by a drive current times the amplitude
of the drive current.

[0022]A preferred embodiment for the first storage device is a DRAM
holding the counts, and an EEPROM holding the decay reference level. The
EEPROM is program at the end of the assembly with the decay date points
measured on actual LED samples of similar product quality. The controller
comprises a microprocessor to perform the data processing and
input/output functions.

[0023]FIG. 2 provides another preferred embodiment, wherein the intensity
drive signal is in a form of digital pulses where the combined total
duration corresponds to the duration, and is proportional to the current.
In this case the digital pulses may be directed registered into the
counter, and the accumulated operation length is recorded in the storage
device 1.

[0024]Various structures may be used to achieve the function of data
counting, storage, and referencing. The application of the principles of
the present invention however is not limited by the specific examples
illustrate herein above. It is conceivable that the circuit blocks may be
combined or separated for various design and cost considerations.

[0025]Although various embodiments utilizing the principles of the present
invention have been shown and described in detail herein, those skilled
in the art can readily devise many other variances, modifications, and
extensions that still incorporate the principles disclosed in the present
invention. The scope of the present invention embraces all such
variances, and shall not be construed as limited by the number of
elements, number of layers, or specific direction and angles.